Solar energy collectors have been used for centuries. Non-imaging solar collectors can be stationary, usually set up facing in a southerly direction in the northern hemisphere to enable absorbing a maximum amount of impinging solar radiation. Imaging solar collectors which track the sun also have a long history. The tracking solar collectors may be classified into two main types: one type includes spherical concentrators having energy absorbing means at the focal point. Either the concentrators or the absorber is controlled to follow the sun by moving on two axes; the other type includes cylindrical concentrators such as trough-like parabolically shaped reflecting collectors which move on one axis to track the sun.
The trough-like parabolic collectors may be mounted in an east-west orientation, that is with its longitudinal axis in the east-west direction and set originally at an angle to the horizontal so that the plane of the open side of the parabola is perpendicular to the sun's rays most of the day. The daily tracking rotation movement accounts for the differences in the azimuth angle of the sun during the year.
The longitudinal axis of the trough-like collector may also be set in a north-south direction wherein the collector rotates on its longitudinal axis through the day to follow the sun as it rises in the east, goes through its zenith and sets in the west.
There are three methods of controlling solar tracking systems in present use. There is a tracking system that uses a sensor to maintain the collector in its desired heliotropic position. There is a tracking system that is controlled by a computer program which takes into account the variations of the sun's position throughout the day and the year. There is also a hybrid system which uses the computer programming system to generally orient the collectors and uses a sensor system for the verniar corrections.
In both the sensor controlled system and the hybrid system the sensor apparatus is the heart of the system. The sensor apparatus in the past has been a critical part of the solar tracking system. Two major sensor devices used in the prior art are optical sensors and flux integration sensors.
The optical sensors generally utilize optical detectors connected in bridge circuitry which provides control signals when there is a lack of balance caused by disorientation of the reflector. Thus where the reflector is not properly positioned the off-center shift generates an error signal. The error signal causes a servo-system to reorient the reflector to correct the error in position.
The most common type of optical sensor in use is the shadow band sensor. In this sensor a partition between detector means is used to cast a shadow on the detectors or arrays of detectors. When the collector is not properly positioned, the shadowed areas are unequal, the circuit is unbalanced and a signal is generated for correcting the error. Precise mechanical alignment of the sensor relative to the reflector is required. In addition the tracking position can be upset by the effects of clouds, for example, or an insect walking across the sensor and causing a shadow on the detector.
Other optical detectors are flux line detectors which are placed on both sides of the focal point or line of the concentrator. The advantage of flux line detectors is the optical gain due to the concentration. However flux line detectors are affected by local perturbations in the concentrator surface.
Other disadvantages of optical sensors are unequal aging of the detetectors, changes in the electronic bias of the detectors, dirt on the photo-cells of the detectors or cover plate and mis-aligment between the sensor and optimum reflector position.
The flux integration sensors are typified by the resistance wire sensors wherein, a fine resistance wire is wrapped in a spiral around the absorber. Heat flux variations caused by solar radiation are sensed by changes in the electrical resistance of the wire. The collector is driven to follow the maximum resistance (most heat).
These sensors are difficult to service or replace because of access problems. The controls are complicated by the necessity of driving the reflector through a range of angles to determine the position of maximum resistance. There is also a basic insensitivity to reflector angular position.